PRIMITIVE VERTEBRATES 295 



is in one piece with a narrow anal fin running forward for a short 

 distance. In front of this the under side of the body is broad, 

 convex, marked by longitudinal pleats, and bounded on each 

 side by a prominent fold which runs forwards to the oral hood. 

 These two lateral folds converge behind and end where the 

 anal fin begins. In the description of the Dog- Fish it was stated 

 (p. 258) that the various unpaired fins are probably expanded 

 surviving fragments of a continuous fin which, in ancestral 

 forms, ran along the middle of the back, round the tail, and 

 forwards for some distance along the under surface. Such a 

 condition is actually represented by the Lancelet, for, as 

 just stated, its unpaired fins constitute a continuous fringe to 

 the body. But further, it is by no means improbable that the 

 pectoral and pelvic fins of an ordinary fish represent the front 

 and back ends of a continuous lateral fin which once existed 

 on either side. There are no paired fins in the Lancelet, but 

 the lateral ridges in front of the anal fin are perhaps equivalent 

 to such continuous lateral fins. 



The Lancelet is obviously segmented, i.e. divided into a 

 number of similar successive parts from before backwards, and 

 this segmentation is well seen in the muscles which make up 

 a great part of the side of the body, these being divided up 

 into > -shaped sections. At first view the animal looks as if it 

 were bilaterally symmetrical, but this is not the case, for the 

 muscle -segments do not correspond on the two sides of the 

 body; and further, the external opening of the intestine (there 

 is no cloaca) is placed on the left side of the body near the 

 base of the tail-fin. There is also a lack of symmetry in other 

 respects which need not be mentioned here. 



A well-developed notochord is present, and like many of the 

 other internal organs can be made out without dissection by 

 examination of small specimens mounted whole as microscopic 

 objects. There is, however, one peculiarity about it. Instead 

 of stopping short about the middle of the brain, as it does in 

 the higher Vertebrates, it runs to the extreme front, and the 

 scientific name of the sub-class alludes to this (Gk. cephalon, 

 head; chords, string). 



No gill-slits are visible on the exterior, but dissection shows 

 that a very large number are present as oblique openings in the 

 wall of the large pharynx. They do not, however, open directly 



